1. Field of the Invention
The present invention relates to an electronically controlled active exhaust-noise attenuation muffler which positively attenuates a noise level of the exhaust system by utilizing a secondary sound source such as a loudspeaker provided in the muffler.
2. Description of the Prior Art
In recent years, there have been proposed and developed various active sound absorbing type mufflers equipped with a secondary sound source such as a loudspeaker provided in the muffler, in order to actively attenuate exhaust noise corresponding to a primary sound by utilizing a secondary sound created by the secondary sound source. Such active sound absorbing type mufflers have been disclosed in Japanese Patent First Publication (Tokkai Heisei) No. 3-174198 and Japanese Utility Model First Publication (Jikkai Heisei) No. 4-11207.
Referring now to FIG. 5, there is shown the conventional active sound absorbing type muffler with a secondary sound source. The active sound absorbing type muffler will be hereinafter abbreviated as an "active muffler". In FIG. 5, an exhaust pipe 11 is connected to a muffler body 13, so as to properly reduce the noise of exhaust gases G flowing therethrough. Reference numeral 15 denotes a tailpipe extending from the muffler body 13 to the rear of the vehicle. A penetration portion of the exhaust pipe 11, which is centrally located in the muffler body 13 in such a manner as to pass through the latter, is formed with perforations 17. In general, the space defined between the outer shell 14 of the muffler body 13 and the centrally located penetration portion of the exhaust pipe 11, is filled with glass wool, steel wool, or some other heat-resistant sound deadner (sound absorption material) 19. A support member 21 is provided between the outer shell 14 of the muffler and the centrally located pipe section in order to support a loudspeaker (as explained later) at a given distance from the outer periphery of the centrally located pipe section. The support member 21 has an opening 25 at which a loudspeaker 27 functioning as a secondary sound source is provided. A microphone 29 is also provided in the centrally located penetration portion of the exhaust pipe 11, to detect the noise of exhaust gases G flowing through the pipe 11, namely a frequency of the exhaust noise and an amplitude of the noise. In actual, an active sound absorbing operation of the typical active muffler is achieved by transmitting a secondary sound which is in opposite phase to the detected noise of the exhaust gas G, by virtue of the speaker 27. In such a conventional active muffler with a secondary sound source, the speaker 27 which serves as a secondary sound source is arranged to emit the secondary sound directly to the exhaust flow in the centrally located pipe section of the exhaust pipe 11, for the purpose of exhaust noise reduction. Therefore, an actual neutral position of the vibrating surface 33 of the speaker 27 would be shifted from a predetermined design neutral point or position, owing to an increase in static pressure P acting on the front side of a vibrating surface 33 of the speaker 27, based on an increase in a flow velocity V of the exhaust gas G as illustrated in FIG. 6. This is because the increase in the flow velocity V of the gas G results in an increase in pressure loss in the exhaust gas flow. Due to the increase in static pressure P, the deviation of the actual neutral position of the vibrating surface 33 relative to the design neutral position, results in a decrease in desired displacement of the vibrating surface 33 of the speaker 27. As a result, a noise reduction performance in the exhaust system may be lowered. In more detail, in the event that two static pressure levels both at the front and rear sides of the vibrating surface 33 of the speaker are balanced to each other, the vibrating surface 33 can oscillate with a maximum permissible amplitude .delta. at an actual vibrating neutral position equivalent to a predetermined design neutral position, as seen in FIG. 7(a). In contrast to the above, in the event that the actual vibrating position of the vibrating surface 33 is slightly displaced from the predetermined design neutral position owing to the increase in static pressure P, the amplitude of the vibrating surface 33 may be undesirably reduced to a value .delta.', as seen in FIG. 7(b). This tends to deteriorate a noise reduction performance of the exhaust system. As is generally known, the previously-noted conventional active muffler is most effective to attenuate a particular exhaust noise which is within a particular frequency range of 20 Hz to 500 Hz. However, upon the engine revolution has reached to a high revolution, a higher-order wave propagation component, particularly a secondary component of the noise of combustion and exhaust of an internal combustion engine can be generated by exhaust flow past a sharp edge in the exhaust train, venturi noise in the carburetor and friction between forceful exhaust flow and respective pipes, namely the exhaust pipe and the tailpipe. In general, such a secondary noise component is a high-frequency noise having frequencies above 500 Hz. The conventional active muffler also suffers from the drawback that the high-frequency noise component which is missed by the prior art active muffler may be emitted from the muffler unit via the tailpipe into the atmosphere at the rear of the car, without any noise reduction effects.